Amide waxes have been used for many years as slip agents in the production of films. Chemically, the waxes are primary, secondary, tertiary, or bis fatty amides, such as oleamide and erucamide.
Amidic waxes are generally immiscible with the host polymer with which they are typically associated via an extrusion process. Host polymers typically include polypropylene, ethylene/alpha olefin copolymer, ethylene/vinyl acetate copolymer (EVA) or other ethylene copolymers such as ionomer resin, ethylene/acrylic or methacrylic acid copolymer, ethylene/acrylate or methacrylate copolymer; or low density polyethylene (LDPE).
Producers of polymeric film operate on the principle that the wax molecule is of a lower molecular weight than the host polymer, and therefore generally more mobile than the host polymer. When the host polymer is in a molten state (during extrusion), waxes can migrate more freely through the host polymer. Waxes migrate through a host polymer in solid state as well, but at a much slower rate. This wax migration, and the polar amide component of the wax molecule, leads to the phenomenon that the wax will diffuse to the surface of the polymeric film. The diffusion rate for surface migration is the “bloom rate”. The bloom rate is influenced not only by the relative molecular weights of the wax and host polymer, but also by the level of saturation of the wax hydrocarbon chain, the polarity of the host polymer, and the architecture of the secondary, tertiary, and bis fatty amide molecules.
Producers of polymeric film also operate on a second principle that packaging applications typically require a film with a low coefficient of friction (COF). This requirement is dictated by the need for the film to run properly on packaging equipment used by food processors and other packagers. For example, in the case of Vertical Form Fill Seal (VFFS) equipment, typical film requirements are a film-to-film COF of less than 0.3. Low molecular weight amidic waxes require sufficient time to “bloom” to the surface before a low COF is achieved in the final film. The resin choice for the surface layer of a VFFS film must have sufficient heat seal properties. Thus, sealant layers typically include those resins mentioned above, such as LLDPE, metallocene catalyzed polymers, and EVA polymers. These materials are much tackier than polypropylene (PP) or propylene/ethylene copolymer (EPC), and present a challenge for the development of adequate film surface properties such as COF or slip.
To help reduce this surface tackiness or tendency for the polymeric film to stick to itself or block, antiblock agents are typically utilized in the surface layer. Typical particulate antiblock agents include various silicas, carbonates, synthetic particulates, etc. Other types of antiblock agents include amidic waxes themselves such as the bis fatty amides. While such antiblock agents can reduce tackiness, they may not yield a high slip polymeric film.
For some packaging applications, the polymeric film is extruded in annular form, one or both edges of the film are slit, and the planar film is either wound or centerfolded in a manner well known in the art. A cylindrical forming piece is used to cast the molten polymer into an initial film thickness and diameter in the shape of a tube. The molten polymer is quenched to solid state by a water bath or spray, as it travels over the forming cylinder, into a continuous length of precursor film tubing.
A problem encountered in this process is that at the extrusion die, a significant amount of wax migrates to the surface of the just emerging tubing as the precursor film contacts film making equipment located downstream of the extrusion die. If wax has been added to the host polymer in the extrusion process at a level to provide the adequate (low enough) COF and adequate (high enough) slip properties needed in the final heat shrinkable film, the equipment becomes covered in wax over time. Such waxy deposits eventually break off in solid form or transfer to the tube in liquid form, resulting in aesthetic anomalies in the final film. Although aesthetic or cosmetic anomalies in the film are sufficient by themselves to affect the commercial viability of the film, at some level of contamination, the physical properties of the film can be compromised as well. Thus, frequent cleaning of the film making equipment may be required, resulting in undesirable downtime. Film end use governs the frequency of the cleaning interval, thereby limiting continuous extrusion to relative short periods of time, and increasing the cost of film production.
A second problem associated with the production of many polymeric films intended for packaging applications is that after the precursor film forming step described above, additional orientation steps are completed and a resulting heat shrinkable film is typically wound up in roll form. Amidic waxes such as erucamide often require a minimum of 24 hours to approach a sufficient equilibrium concentration on the film's outer surface (i.e. often require a minimum of 24 hours to bloom to the film's outer surface) to permit adequate processing on packaging equipment. When polymeric films are made, especially heat shrinkable films, the film is rolled up such that the bottom (that is, the interior winds near the film core) of a film roll can be under a relatively high tension of several hundred pounds per square inch. This roll pressure has been found to retard or even prevent significant wax from blooming to the film surface after the film is wound into roll form. Additionally, a film roll which is wound under high pressure may result in wax diffusion through several film layers thereby depleting wax concentration in some regions of the roll and buildup in other regions. Wax migration in the roll undermines desired consistent film performance from the beginning to the end of the roll. Consequently, the level of wax on the film surface has been shown to be greater on the top of the roll (i.e. the exterior winds of the roll) where roll pressure is less, than it is at the end of a roll nearest the metal, paper or plastic roll core member. This significantly affects the ultimate performance of the film, especially with respect to the manner in which the film runs or “tracks” on high-speed packaging equipment. Additionally, in some cases only portions of a roll of film may be utilized on packaging equipment, and the rest has undesirably “blocked”, i.e. bonded together at film surfaces on the roll, resulting in significant roll footage waste.
Applicants have found that a solution to the above-mentioned first problem (the build up of wax during the film making process) is to dispose one or more amidic waxes in an inner layer of a multi-layer film. This solution results in either no amidic wax present in the surface layers of the multi-layer film, or else significantly less wax present in the surface layers than in one or more inner substrate layers during the initial stages of film formation.
Applicants have found that a solution to the second above-mentioned problem (the differential level of surface wax within the rolled film) is to include a transition metal salt of stearic acid, or an ester of stearic acid, in the inner substrate layer or layers, and/or the surface layers of the film. Applicants have found that a transition metal salt of stearic acid, or an ester of stearic acid, acts as a facilitator to cause the wax to bloom more quickly to the surface of the film. Such accelerated bloom provides a multi-layer film with desired surface properties. Thus, wax in an inner substrate layer diffuses to the film's outer surfaces before the film is wound onto a roll, thereby providing adequate slip prior to the pressures encountered during winding of the film onto a core member. Such controlled bloom time can reduce the need for above-mentioned wax migration within a film roll and result in more uniform levels of wax at the film surface throughout a roll length.
The two solutions in suitable combination provide a polymeric multi-layer film, such as a heat shrinkable film, having good slip (i.e. low COF) properties, while eliminating significant wax build-up on film-making machine components. The ability to control wax migration reduces manufacturing waste and provides improved performance by ensuring consistency throughout rolls of polymeric films.
Definitions
“Polymeric film” herein means a thermoplastic polymeric film, laminate, sheet, web, or the like, which can be used to package an article. The film can be used alone, or as a component in a rigid, semi-rigid, or flexible product, and can be adhered to a non-polymeric or non-thermoplastic substrate such as paper or metal.
“Polymer” and the like herein means a homopolymer, but also copolymers thereof, including bispolymers, terpolymers, etc. Polymers that may be used to formulate outer, substrate, or core film layers include, by way of example, any resin typically used in films for packaging applications, such as the following polymers, their copolymers, or blends: polyolefin, polyethylene, ethylene/alpha olefin copolymer, ethylene/vinyl acetate copolymer; ionomer resin; ethylene/acrylic or methacrylic acid copolymer; ethylene/acrylate or methacrylate copolymer; low density polyethylene, polypropylene, polystyrene, polycarbonate, polyamide (nylon), acrylic polymer, polyurethane, polyvinyl chloride, polyvinylidene chloride, polyester, ethylene/styrene copolymer, norbornene/ethylene copolymer, ethylene/vinyl alcohol copolymer, etc. Additives can be included in the composition to impart properties desired for the particular article being manufactured. Such additives include, but are not limited to, fillers, pigments, dyestuffs, antioxidants, stabilizers, processing aids, plasticizers, fire retardants, anti-fog agents, etc.
“Polyolefin” herein means a homopolymer, copolymer, terpolymer etc. of a C2 to C10 aliphatic alpha-olefin, such as ethylene, propylene, butylene, hexene, octene, norbornene, and the like. “Ethylene/alpha-olefin copolymer” (EAO) herein refers to copolymers of ethylene with one or more comonomers selected from C3 to C10 alpha-olefins such as propene, butene-1, hexene-1, octene-1, etc. in which the molecules of the copolymers comprise long polymer chains with relatively few side chain branches arising from the alpha-olefin which was reacted with ethylene. This molecular structure is to be contrasted with conventional high pressure low or medium density polyethylenes which are highly branched with respect to EAOs and which high pressure polyethylenes contain both long chain and short chain branches. EAO includes such heterogeneous materials as linear medium density polyethylene (LMDPE), linear low density polyethylene (LLDPE), and very low and ultra low density polyethylene (VLDPE and ULDPE), such as DOWLEX™ or ATTANE™ resins supplied by Dow, ESCORENE™ or EXCEED™ resins supplied by Exxon; as well as linear homogeneous ethylene/alpha olefin copolymers (HEAO) such as TAFMER™ resins supplied by Mitsui Petrochemical Corporation, EXACT™ resins supplied by Exxon, or long chain branched (HEAO) AFFINITY™ resins supplied by the Dow Chemical Company, or ENGAGE™ resins supplied by DuPont Dow Elastomers. EAO also includes IPN resins, such as Elite™ resins supplied by Dow.
“Host polymer or host resin” herein mean polymers that may be used to formulate the outermost and substrate layers of the packaging films of the invention and which house the amidic waxes. Host polymers that may be used typically include the following polymers, their copolymers or blends: polyethylene, ethylene/alpha olefin copolymer, ethylene/vinyl acetate copolymer, ionomer resin, ethylene/acrylic or methacrylic acid copolymer, ethylene/acrylate or methacrylate copolymer, low density polyethylene, high density polyethylene, polypropylene, propylene/ethylene copolymer, propylene/ethylene/butene terpolymer, polystyrene, syndiotactic polystyrene, ethylene/styrene copolymer, and norbornene/ethylene copolymer.
“Core layer” herein means a central layer(s) of a multi-layer film; in a three layer film, the core layer is the second layer in the sequence of layers; in a five layer film, the core layer is the third layer in the sequence of layers.
“Substrate layer” herein means an inner host layer of a multi-layer film, not an outermost layer of the film; in a three layer film, the substrate layer is disposed between the respective outermost layers; in a four layer film, the substrate layers are the second and third layers in the sequence of layers and disposed between the respective outermost layers; in a five layer film, the substrate layer(s) denote the host layer(s) disposed between outermost layers.
“Interpenetrating Network Polymer” (IPN resin) herein refers to multicomponent molecular mixtures of polymer chains. Because of molecular mixing, IPN resins cannot be separated without breaking chemical bonds. Polymer chains combined as IPN resins are interlaced at a molecular level and are thus considered true solid state solutions. Interpenetrating networks, unlike blends, become new compositions exhibiting properties distinct from parent constituents. Interpenetrating networks provide phase co-continuity leading to surprising enhancement of physical properties. Due to the mixture of at least two molecular types, these compositions may exhibit bimodal or multimodal curves when analyzed using TREF or CRYSTAF. Interpenetrating networks as herein used includes semi-interpenetrating networks and therefore describes crosslinked and uncrosslinked multi-component molecular mixtures having a low density fraction and a high density fraction. Specific production methods for preparing IPN resins useful for carrying out the present invention, are disclosed in U.S. Pat. No. 5,747,594 (deGroot et al.), U.S. Pat No. 5,370,940 (Hazliff et al.), and WO 94/17112 (Kolthammer), all herein incorporated by reference in their entirety. IPN resins can be prepared using a parallel or sequential multiple reactor scheme, and can be produced from a solution polymerization scheme. Alternatively, IPNs useful for the inventive films may be prepared within a single reactor by completing polymerization of the heterogeneous component prior to initiating the polymerization of the homogeneous component. Examples of catalysts suitable for producing the homogeneous component are described in U.S. Pat. Nos. 5,026,798 and 5,055,438 (Canich); U.S. Pat. No. 3,645,992(Elston); U.S. Pat. No. 5,017,714 (Welborn); and U.S. Pat. No. 4,076,698 (Anderson); all herein incorporated by reference in their entirety.
“Homogeneous” herein refers to polymerization reaction products of relatively narrow molecular weight distribution (Mw/Mn less than or equal to 3.0) and relatively narrow composition distribution. Homogeneous polymers are structurally different from heterogeneous polymers, in that homogeneous polymers exhibit a relatively even sequencing of comonomers within a chain, a mirroring of sequence distribution in all chains, and a similarity of length of all chains, i.e., a narrower molecular weight distribution. Furthermore, homogeneous polymers are typically prepared using metallocene or other single-site catalysts, rather than, for example, Ziegler Natta catalysts. Processes for preparing and using linear homogeneous polyolefins are disclosed in U.S. Pat. No. 5,206,075, U.S. Pat. No. 5,241,031, PCT International Application WO 93/03093, U.S. Pat. Nos. 5,026,798 and 5,055,438 (Canich); U.S. Pat. No. 3,645,992(Elston); U.S. Pat. No. 5,017,714 (Welborn); and U.S. Pat. No. 4,076,698 (Anderson); each of which is hereby incorporated by reference thereto, in its entirety. Further details regarding the production and use of linear homogeneous ethylene/alpha-olefin copolymers are disclosed in PCT International Publication Number WO 90/03414, and PCT International Publication Number WO 93/03093, both of which designate Exxon Chemical Patents, Inc. as the Applicant, and both of which are hereby incorporated by reference thereto, in their respective entireties. Still another genus of homogeneous polyolefins is disclosed in U.S. Pat. No. 5,272,236, to LAI, et. al., and U.S. Pat. No. 5,278,272, to LAI, et. al., both of which are hereby incorporated by reference thereto, in their respective entireties. Each of these patents disclose “substantially linear” homogeneous long chain branched ethylene/alpha-olefin copolymers produced and marketed by The Dow Chemical Company. Still another genus of homogeneous polyolefins is homogeneous hyper-branched polyolefins, which is inclusive of SSH polyolefin. The phrase “substantially spherical homogeneous polyolefin” (i.e.,“SSH polyolefin”) refers to single site catalyzed resin with a polymer architecture wherein there are at least 50 side branches, such as at least 60 side branches, from the main chain for every 1000 main chain carbons. Hyper-branched homogeneous polyethylene, while resembling other homogeneous resins in aspects such as low polydispersity index (Mw/Mn of less than or equal to 3.0, such as less than 2.7, or having a Mw/Mn of from 1.9 to 2.5), do not resemble commercial linear homogenous ethylene/alpha-olefin copolymer, or long chain branched homogeneous ethylene/alpha-olefin copolymer, at least in that the hyperbranched homogeneous polyethylenes may be characterized as having a mixed population of side chains of different chain length, together with a high side chain branching level. Additionally, it is possible that at least some of the short chain side branches of the hyperbranched homogeneous polyethylene are themselves short chain branched. While the molecular weight is similar from one polymer chain to the next, the different side branch length and/or structure affects the crystallinity in a different manner than commercial homogeneous resins such as Mitsui TAFMER® polymer or Dow ENGAGE® polymer.
“Heterogeneous” herein refers to polymerization reaction products of relatively wide variation in molecular weight (Mw/Mn greater than 3.0) and relatively wide variation in composition distribution, i.e., typical polymers prepared, for example, using conventional Ziegler-Natta catalysts. Heterogeneous copolymers typically contain a relatively wide variety of main chain lengths and comonomer percentages. Processes for preparing heterogeneous geneous resins are described in U.S. Pat. No. 4,314,912 (Lowery et al.), U.S. Pat. No. 4,547,475 (Glass et al.), and U.S. Pat. No. 4,612,300 (Coleman, III); all incorporated herein by reference in their entirety.
“Transition metal salt of stearic acid, or ester of stearic acid” herein means a transition metal salt of stearic acid, such as zinc stearate; or an ester of stearic acid, such as glycerol monostearate; or a blend thereof.
“Antiblock agents” herein means particles which may be used to lower the coefficient of friction and/or adhesion of outer film surfaces, and includes various silicas (fumed, precipitated, gelled, etc.), natural silicates (talc, diatomaceous earth, etc.), magnesium silicate, carbonates, synthetic silicate, natural alumina; synthetic alumina, alumino silicate, synthetic particulates (uncrosslinked styrenic polymeric particles, crosslinked styrenic polymeric particles, high molecular and ultrahigh molecular weight siloxanes, uncrosslinked acrylic polymeric particles, crosslinked acrylic polymeric particles, polyethylene particles, styrenic, acrylic, siloxane, fluoropolymer, etc.), etc. Other types of antiblock agents include amidic waxes such as the bis fatty amides. Antiblock quantities depend at least in part depend on particle size. Generally, the larger the particle, the less needed. When the size is less than one micron, for example, 20,000 ppm provides adequate slip properties, whereas for a particle size of about 5 μ, about 1000 ppm antiblock loading provides the same effect. Specific methods for producing films having good antiblocking performance are disclosed in U.S. Pat. No. 5,925,454 (Bekele), herein by reference in its entirety.
“Heat shrinkable” herein refers to a property of a material which, when heated to a temperature of 200° F., will exhibit a free shrink (ASTM D 2732-83) of at least 8% in the longitudinal direction, and at least 8% in the transverse directions of the film. Heat shrinkable films of this invention are solid state oriented as contrasted to hot blown films which are melt state oriented. Heat shrinkable films exhibit free shrink at their softening point as contrasted to hot blown (single bubble) film which do not show shrink performance at low temperatures and must be heated to temperatures approaching the melting point of film resins.
“Amidic wax” herein refers to primary, secondary, tertiary, or bis(fatty) amides. Examples of the different types include primary fatty amides such as erucamide, behenamide, oleamide, or stearamide; secondary fatty amides such as stearylerucamide, erucylerucamide, oleylpalmitamide, stearylstearamide, or erucylstearamide; tertiary fatty amides such as dimethylstearamide, diethylstearamide; and N,N′-bis(fatty) amides such as N,N′-ethylene bis(stearamide), N,N′-methylene bis(stearamide), N,N′-ethylene bis(oleamide), or N,N′-propylene bis(oleamide). These waxes can be used with the present invention singularly or in combination.
All compositional percentages used herein are presented on a “by weight” basis, unless designated otherwise.